A magnetic hard disk drive (hereinafter also referred to as HDD) includes a rotating magnetic disk as a recording medium, a recording/reproducing head supported by a suspension arm, and an actuator for actuating the suspension arm. Magnetic information recorded on the magnetic disk is read with a magnetic sensor in the reproducing head. As the magnetic sensor for the reproducing head, a magnetoresistive sensor such as a GMR sensor or a TMR sensor has been conventionally used.
The magnetic recording density in each HDD has been increasing year by year. The highest surface recording density of HDDs currently on the market is approximately 600 Gbits/in2. According to the HDD technology road map, the surface recording density will reach 1 Tbit/in2 around the year 2013, and to 2 Tbits/in2 around the year 2015.
To increase a surface recording density is to reduce the size of recording bits in the magnetic disk, and the size of the magnetoresistive sensor needs to be reduced accordingly. Therefore, the track width and the distance between shields in each magnetoresistive sensor are reduced. However, it is considered that, if the size of each magnetoresistive sensor is further reduced in the future, thermal magnetic noise due to fluctuations caused by the heat of magnetization in the magnetic material will increase, and a practical SN (Signal-to-Noise) ratio in reproduced signals will not be maintained.
To avoid the problem of thermal magnetic noise, there have been known reproducing heads each including a spin-torque oscillator.
A spin-torque oscillator (also referred to as STO) has a fundamental structure that is a film stack formed by stacking a free layer, a nonmagnetic spacer layer, and a pinned layer (a magnetization pinned layer). By applying current to the STO, the magnetization of the free layer enters a steady oscillatory state due to spin-polarized current. The above described fundamental film structure is the same as that of a CPP (Current Perpendicular to Plane)-GMR (Giant Magneto-Resistive) head and that of a TMR (Tunnel Magneto-Resistive) head, and an output of the STO derives from a magnetoresistive effect. Therefore, the STO outputs a high-frequency signal in accordance with oscillations of the magnetization of the free layer. That is, the STO is an oscillator that outputs an oscillation voltage deriving from oscillations of the magnetization of the free layer. In view of this, the free layer of an STO is also called an oscillating layer.
In a reproducing head including a spin-torque oscillator (hereinafter also referred to as the STO reproducing head), an STO is used as the magnetic sensor. By using the fact that the amplitude and frequency of magnetization oscillations in the free layer in an STO depend on an external magnetic field acting on the STO, changes in the amplitude of magnetization oscillations or changes in frequency or phase due to a medium magnetic field generated from medium bits are detected, and magnetic information is read. The magnetization oscillations in an STO are induced by applying current. Therefore, it is considered that, when the magnetization oscillation energy of the STO is sufficiently larger than the thermal energy, fluctuations of the magnetization due to heat are relatively restrained, and it is possible to achieve a sufficiently higher SN ratio than that achieved by a reproducing method using a conventional magnetoresistive sensor. There is a known technique suitable for achieving a higher SN ratio and perform higher-speed reproduction in a case where changes in frequency or phase are detected than in a case where changes in oscillation amplitude are detected. In view of this, it is considered that the problem of thermal magnetic noise can be avoided by using an STO as the magnetic sensor.
Other than the above-mentioned problem of thermal magnetic noise, an increase in resolution also, causes a problem in increasing the recording density. When a reproducing head accesses a bit from which information is to be read (a target bit), not only the magnetic field from the target bit but also the magnetic fields from the adjacent medium bits act on the reproducing head, resulting in a low accuracy in the information reading. Therefore, the distance between magnetic shields in each reproducing head using a magnetoresistive sensor is made shorter, to restrain interferences between bits and achieve a higher resolution. This measure is considered effective in STO reproducing heads, and there have been known reproducing heads each having an STO interposed between two shield films.
In each conventional STO reproducing head, however, the fundamental film structure of the STO is the same as that of a CPP-GMR head or a TMR head, and it is difficult to reduce the intershield distance to 15 nm or shorter due to a requirement for the film thickness of the STO. Therefore, it is considered difficult to use such an STO reproducing head to read information from a medium of 4 Tbits/in2 or higher in terms of resolution. The requirement for the film thickness of an STO is set for the following reasons. If the film thickness of the antiferromagnetic layer for pinning the magnetization of the pinned layer that accounts for a large portion of the entire film thickness becomes 5 nm or smaller, the unidirectional magnetic anisotropic constant becomes rapidly lower, and the magnetization of the pinned layer is not pinned. Therefore, the film thickness of the antiferromagnetic layer needs to be greater than 5 nm.
In a case where an STO is used as a magnetic sensor, the problem of thermal magnetic noise due to miniaturization of the device can be more effectively avoided than in a case where a conventional CPP-GMR head or TMR head is used. As for higher resolutions, however, an STO magnetic sensor has the same problem as that of a conventional CPP-GMR head and TMR head, and there is a demand for a high-output and high-Q STO that has a small film thickness and is suitable for high-density magnetic recording and reproduction.